1. Field of the Invention
The present invention relates to a monitoring method used to monitor an optical output of a vertical cavity surface emitting laser (VCSEL) array and a VCSEL array on which monitoring is performed.
2. Description of the Related Art
A VCSEL is a type of semiconductor laser that is designed to emit light at right angles with respect to a semiconductor substrate. A number of such VCSEL lasers can be integrated and arranged in a two-dimensional array. When the VCSEL array is applied as a light source for exposure in an electrophotography apparatus, parallel processing on a printing operation using multiple beams can be achieved, thereby improving resolution and increasing print speed.
If the VCSEL is used as the light source of an electrophotography apparatus, such an apparatus detects an optical output obtained from the VCSEL with an optical sensor and controls the driving voltage (the driving current) of the VCSEL on the basis of the detection result.
U.S. Pat. No. 5,809,050 (p. 12 and FIG. 8B) describes an example monitoring apparatus of the related art. The monitoring apparatus has a VCSEL and an optical sensor that are included in the same package, detects a part of a laser beam emitted from the VCSEL, and monitors the light quantity of that part of the laser beam. The monitoring apparatus disclosed in U.S. Pat. No. 5,809,050 (p. 12 and FIG. 8B) is described below with reference to FIG. 9 of the present application.
A VCSEL 710 and an additional laser structure 720 are electrically isolated from each other by a reduced-conductivity material 730. The VCSEL 710 is structurally similar to the additional laser structure 720. When subject to a reverse bias opposite in polarity to the electrical potential of the driving current of the VCSEL 710, the additional laser structure 720 operates as an optical sensor.
Electrodes 740 and 741 are formed to overlap the VCSEL 710, and electrodes 745 and 746 are formed to overlap the optical sensor. A first cladding layer 750 overlies the electrodes 741 and 745. A core layer 760 overlies the first cladding layer 750. A second cladding layer 770 is stacked on the core layer 760. The core layer 760 has a diffraction grating 780. The refractive index of the core layer 760 is greater than that of the first cladding layer 750 and that of the second cladding layer 770. Therefore, light can be confined within the core layer 760.
A laser beam 701 generated by the VCSEL 710 travels in the core layer 760. The laser beam 701 partially passes through the core layer 760 and becomes an output light beam 702. The remainder of the laser beam 701 is diffracted by the diffraction grating 780 and becomes diffracted light 703. The diffracted light 703 travels along an optical waveguide formed by the first cladding layer 750, the core layer 760, and the second cladding layer 770, and then enters the additional laser structure 720. The additional laser structure 720 operates as an optical sensor when subject to a reverse bias, and a current is generated from the additional laser structure 720 on the basis of the intensity of the diffracted light 703. A control circuit 790 operates in response to the current supplied from the additional laser structure 720 via a conductor 791, and feeds a current to the VCSEL 710 via a conductor 792. The VCSEL 710 can control the output light beam 702 to have a predetermined intensity by controlling this current.
If the VCSEL array is assumed to be used as a light source of the electrophotography apparatus, it is required to monitor each of light-emitting devices arranged in an array.
For example, if the VCSEL array is used as the light source of the electrophotography apparatus, even if the currents flowing through the devices are equal to one another, optical outputs obtained from the devices may be different due to changes over time.
Depending on the printing pattern being used, a driving period for a certain light-emitting device (a device A) can be different from that for another light-emitting device (a device B). Thus, when the driving period for the device A is longer than that for the device B, a quantity of heat generated from the device A is greater than that generated from the device B. The optical output of the VCSEL is affected by heat, and thus even if equal currents are fed to the devices A and B, their optical outputs may be different from each other.
Therefore, the light-emitting devices arranged in the array need to be individually monitored.
However, if the above-discussed monitoring apparatus disclosed in U.S. Pat. No. 5,809,050 is applied to a VCSEL array, and the light-emitting devices are individually monitored, as many optical sensors as there are the light-emitting devices need to be additionally provided. Thus, the complexity of the apparatus is increased.